Processing physiological fluids by centrifugation for separating the fluids into components of different densities is known. Physiological fluids include, for example, peripheral blood, umbilical cord blood, and bone marrow aspirate and ordinarily include cellular components. The physiological fluids subjected to the processes described herein may be obtained directly from a patient being treated, in which case the fluids are autologous, obtained from a donor, or obtained from a plurality of donors, in which case the fluids are homologous. While the objects of the invention are primarily concerned with the treatment of human fluids, it will be appreciated that the methods and apparatus described herein are equally applicable to fluids from other species.
A primary objective of the invention is to isolate and obtain a layer of cells that forms during centrifugation and includes, among other components, platelets, white cells, stem cells, and nucleated cells. This layer is known as the buffy coat, and its density is between that of the red blood cells (1.08-1.09) and that of plasma (1.017-1.026). Plasma with most of the cellular components removed is known as platelet poor plasma (PPP), while plasma with its cellular components is known as platelet rich plasma (PRP). Platelet rich plasma has been found to produce several beneficial effects, such as a more rapid healing of wounds. Thus, another objective of the invention is to provide plasma with an increased level of platelets. This is known as a platelet concentrate (PC) or more broadly as a cell concentrate (CC). A typical concentration is four or more times the native concentration, and a typical ratio of input volume to cell concentrate volume is 6:1. Platelet or cell concentrates obtained by the invention comprises the buffy coat and plasma and may include a small amount of red blood cells.
One of the problems addressed by the present invention is that the specific proportion of the various components and, even, the density of the cells themselves are unique to the particular donor, which precludes an exact a priori determination of the location of any given component in the fluid after centrifugation. For example, the proportion of red blood cells in blood, the hematocrit, varies with each patient, and the average density of the red blood cell component varies with its proportion of neocytes, young red blood cells, whose density is less than 1.08.
Furthermore, the particular technique used to collect the fluids impacts the density of the cells. An anticoagulant is typically added to blood as it is collected, and the amount of anticoagulant and the particular anticoagulant used affects the density, particularly, of red blood cells. This is termed the lesion of collection and results from the effect of the anticoagulant on the osmolarity of the cells. For example, when the anticoagulant is acid citrate dextrose, ACD, red cells become hypo-osmolar and the cells draw water through the dell membrane, which decreases the density of the cells. Other anticoagulants, such as tri-sodium citrate at a concentration of 3.8%, are somewhat hyper-osmolar, which results in shrinkage of the red blood cells and an increase in their density. CPD is iso-osmolar and has much less effect on the density of the cells. CPD and tri-sodium phosphate are preferred and have produced superior results in separations of the kind contemplated herein.
A further factor is that the layers of components form along the radius of centrifugation and are thus cylindrical, which complicates the design of structural elements for separating or collecting the layers.
A system for separating blood into components for producing a platelet concentrate is described in U.S. Pat. No. 6,398,972. The system described in that patent uses a disposable processing unit having two chambers. Blood is drawn into a known syringe and expressed from the syringe into a first chamber of the processing unit. The processing unit is then placed in a centrifuge designed to automatically transfer supernatant fluids from one chamber to another. After a first centrifugation, platelet rich plasma is transferred into the second chamber, and the centrifuge is operated a second time to separate platelets from platelet poor plasma. While this system has many advantages, it has the disadvantage that the blood must be transferred from the syringe to the processing unit, and the centrifuge and the orientation of the processing unit must be controlled to decant the platelet rich plasma to the second chamber.
The first chamber of the system described in the '972 patent includes a disk that is positioned generally at the intersection of the red blood cells and the plasma to prevent decanting of red blood cells into the second chamber.
U.S. Pat. No. 5,456,885 shows a system wherein a collection tube is placed directly in a centrifuge to allow separation of the components. A floating element assumes a position between the plasma and the red blood cells and also acts as a check valve when the lighter phase is expressed from the tube. Systems of this type are, however, not generally capable of separating the buffy coat from the platelet-poor plasma and the red blood cells.
The known centrifuges operate according to a particular protocol when it is desired to obtain a component of intermediate density. For example, when the object is to obtain platelets, it is known to subject blood to a first centrifugation to separate heavier components, such as red blood cells, from plasma, transferring the plasma to a second container or chamber by decanting and then subjecting the plasma to a second centrifugation to separate the plasma from the platelets. The platelets are then separated from the plasma in a second decanting step.
Known techniques for obtaining the desired component of intermediate density are complicated because they require multiple centrifugations and multiple decant or centrifugal transfer steps. Also, the separation of a single component is often complicated because the physical, fluid properties of the desired component may tend to cause it to mix with the other components.
The buffy coat layer is easily disrupted, and when one attempts to express platelet poor plasma through the tip of a syringe, the buffy coat often mixes with the plasma or with the red blood cells. This effectively prevents the expressing of the buffy coat layer either by itself or with only a negligible amount of the other components.
As well, known tubes or syringes designed to be supplied directly to a centrifuge are difficult to use effectively.